Much recent research has been dedicated to improving charged-particle-beam projection-exposure apparatus (charged-particle-beam microlithography apparatus) so as to produce an improved resolution of the transferred pattern and improved "throughput" (i.e., productivity). One example of such apparatus includes a "batch-transfer" apparatus, in which an area representing one "die" (i.e., a pattern equivalent to one of several integrated circuits or other devices that are to be formed on a substrate that is sensitive to the charged particles) or an area representing multiple dies is "transferred" (i.e., projection-exposed) from the mask to the sensitive substrate in one exposure. Unfortunately, preparing the necessarily large mask for such an apparatus is difficult. Also, it is difficult to adequately control aberrations in the charged-particle-beam imaging optical system (hereinafter, simply referred to as the "imaging system") of such apparatus below a specified value at any location within the large optical field required to simultaneously expose an entire die or multiple dies.
Charged-particle-beam microlithography apparatus have been developed in which the mask pattern to be transferred to the sensitive substrate is partitioned into multiple "mask subfields" that are each smaller than one die. The partitioned pattern is transferred to the sensitive substrate one subfield at a time to corresponding "transfer subfields" on the substrate. Such apparatus offer better correction of such aberrations such as displacement of the focal point of each subfield or distortion of the transferred image. Thus, exposure of the substrate can be accomplished with better resolution and positional accuracy across an optically wider field than with batch-transfer apparatus.
A conventional charged-particle-beam microlithography apparatus employing, e.g., an electron beam and used with a conventional partitioned mask forms a "crossover image" of the electron-beam source downstream of the mask. If there are no aberrations in the imaging system, the shape of the crossover image is normally circular and the center of the crossover image is normally located on the optical axis of the electron-beam imaging system.
FIG. 1(a) shows an ideal condition of a crossover image 20. The crossover image 20 is formed on the optical axis A inside and concentric with an aperture 21 defined by an aperture stop 22. The crossover image 20 ideally has a circular shape, as shown in FIG. 1(b). (The aperture stop 22 is employed primarily when a so-called scattering mask is used.)
A scattering mask for an electron beam defines the pattern in part with electron-transmitting portions and electron-scattering portions. An electron-transmitting portion is typically a thin film of, e.g., silicon nitride (SiN), and an electron-scattering portion is typically a tungsten film or other suitable metal. The aperture stop 22 blocks extraneous electrons scattered by the scattering mask.
Further with respect to a conventional apparatus, if a mask subfield through which the electron beam is being transmitted is laterally displaced a large distance from the optical axis A, illumination of that particular subfield requires that the electron beam be greatly deflected from the optical axis A. As a result, the path of the electron beam is much different from when the electron beam passes through a mask subfield that is on or much closer to the optical axis A. When a greatly deflected electron beam is focused by the imaging system, aberrations can arise in the crossover image such as defocus and/or astigmatism. Such aberrations can arise even if the crossover image is focused on the optical axis A. An example of such a crossover image 23 is shown in FIG. 2(a). The transverse profile 24 of the electron beam on the aperture 21 of the aperture stop 22 is fuzzy, as shown in FIG. 2(b).
FIG. 3(a) shows a crossover image 25 experiencing astigmatism. With conventional apparatus, whenever it is necessary to impart a large amount of deflection to the electron beam to project a particular mask subfield onto the substrate, the center of the crossover image will tend to be shifted laterally from the optical axis A. Projection of such a crossover image 25 on the aperture 21 can result in the crossover image having an elliptical profile, as shown in FIG. 3(b), even if the crossover image is focused on the optical axis A, as indicated by the crossover image 26 in FIG. 4.
Thus, the amount of the electron beam that actually passes through the aperture 21 can change with a change in the transverse profile of the crossover image accompanying a change in the amount of deflection of the beam from the optical axis required to image a particular sub-field on the mask. This, in turn, causes undesirable changes in the brightness and contrast of the subfield images on the sensitive substrate, which causes decreased dimensional accuracy and resolution of the projected patterns after the sensitive substrate is developed.
Other conventional apparatus lack an aperture stop 22. Such apparatus include several focusing lenses situated within the imaging system. The focusing lenses provide fine adjustment of the focal-point position of the image on the surface of the sensitive substrate, and energization of these lenses is adjusted during focusing. The crossover image is normally positioned at the center of the several focusing lenses along the optical axis. Otherwise, the size of the image or the beam intensity at the exposed surface could change when the focal point is dynamically or statically adjusted by such lenses. However, whenever the position of the crossover image is changed according to the amount of deflection of the electron beam needed to expose a particular mask subfield, the aforementioned conditions are not met and the size of the image and/or the beam intensity at the exposed surface will be changed.
In cases where no special focusing lenses are provided, focusing is accomplished by adjusting the energization of the imaging system itself. However, there are problems in these cases as well with the size of the image and/or the beam intensity at the exposed surface changing when the position of the crossover image changes.
FIG. 5(a) is an enlarged plan view of a portion of a conventional partitioned mask used by a conventional charged-particle-beam microlithography apparatus. The mask 30 is partitioned into multiple mask subfields 31 by struts 32 serving as boundaries forming a two-dimensional grid. Each mask subfield 31 is, e.g., (1 mm).sup.2 in area. FIG. 5(b) is a section along the A--A line of FIG. 5(a), showing that the struts 32 are dimensioned sufficiently to support the weight of the mask 30.
Each mask subfield 31 includes a centered square patterned region 33. The electron beam passes at least through the patterned region 33. A skirt region ("skirt") 34 extends circumferentially between the patterned region 33 and the struts 32. The skirt 34 blocks transmission of the electron beam. An irradiation region 35 (actually irradiated by the electron beam) is larger in area than the patterned region 33, but normally does not extend past the skirt 34. Thus, only the pattern inside the patterned region 33 inside the skirt 34 is transferred to the sensitive substrate. The electron-beam illumination system projects an image of a specific aperture onto the mask 30. The aperture image ideally extends across the irradiation region 35.
In a conventional apparatus employing a partitioned mask, the electron beam is deflected to sequentially transfer a row of mask subfields extending nearly across the optical axis of the optical system in a specific direction. In order to "transfer" (i.e., project an image onto the substrate) the patterned regions in mask subfields in areas perpendicular to the specific direction, the mask and substrate are scanned during exposure by a stage system in a direction perpendicular to the specific direction.
Consequently, since the mask 30 in FIG. 5(a) is intended to be scanned during exposure, e.g., in a direction as indicated by the arrow 36, the irradiation region 35 is typically larger than the patterned region 33 by a specific amount so that the patterned region 33 does not shift away from the irradiation region 35 during transfer.
If the patterned regions of sequential mask subfields are transferred side-by-side onto the substrate, there is a risk of "stitching errors" developing at the boundaries of corresponding transfer subfields on the substrate.
In instances in which, e.g., the irradiation region 35 in FIG. 5(a) is defocused and becomes larger (i.e., now extending to the border 37) the skirt 34 must be widened in advance so as to extend past the border 37. However, since the skirt 34 does not contribute any pattern for transfer, the skirt 34 is a "useless" area that simply enlarges the area of the mask 30 and, consequently, the mask stage, thereby resulting in increased cost. In addition, whenever the shape of the irradiation region 35 changes or undergoes a shift in position, some of the patterned region 33 shifts away from the irradiation region 35, resulting in portions of the pattern on the mask subfield not being transferred to the corresponding transfer subfield on the substrate. Furthermore, whenever the irradiation density varies in certain mask subfields, the intensity of exposure at the corresponding transfer subfields on the sensitive substrate changes accordingly.
Furthermore, if the shape, position, or irradiation density, etc. of the irradiation region 35 changes in a mask subfield 31 that is separated from the optical axis, problems arise because projected images of the patterns in respective mask subfields transferred onto the sensitive substrate can exhibit overlapping transfer-subfield outlines. In efforts to improve the level of integration of semiconductor devices made using charged-particle-beam microlithography apparatus, such seam overlap is a problem. Problems also arise with irregularities in the exposure amount over the area of each die on the sensitive substrate as the result of changes in the shape, etc., of the irradiation region.